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Key Establishment

Symmetric key problem: How do two entities establish shared secret key in the first place? Solutions: Deffie-Hellman trusted key distribution center (KDC) acting as intermediary between entities. Public key problem:

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Key Establishment

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  1. Symmetric key problem: How do two entities establish shared secret key in the first place? Solutions: Deffie-Hellman trusted key distribution center (KDC) acting as intermediary between entities Public key problem: When Bob obtains Alice’s public key (from web site, e-mail, diskette), how does he know it is Alice’s public key, not Trudy’s? Solution: trusted certification authority (CA) Key Establishment

  2. Deffie-Hellman Key Exchange prime number p, base g secret integer a secret integer b ga mod p gb mod p (ga mod p)b mod p (gb mod p)a mod p Key: (gb mod p)a mod p= (ga mod p)b mod p

  3. Deffie-Hellman Key Exchange: Example • Prime number p=23, base g=5. • Alice: a=6 • Send Bob: g^a mod 23 = 8. • Bob: b=15 • Send Alice: g^b mod 23 = 19. • Alice compute: 19^6 mod 23 = 2 • Bob computer: 8^15 mod 23 = 2

  4. KB-KDC KX-KDC KY-KDC KZ-KDC KP-KDC KB-KDC KA-KDC KA-KDC KP-KDC Key Distribution Center (KDC) • KDC: server shares different secret key with each registered user (many users) • Alice shares a key with KDC: KA-KDC • Bob shares a key with KDC: KB-KDC KDC

  5. Key Distribution Center (KDC) Q: How does KDC allow Bob, Alice to determine shared symmetric secret key to communicate with each other? KDC generates R1 KA-KDC(A,B) KA-KDC(R1, KB-KDC(A,R1) ) Alice knows R1 Bob knows to use R1 to communicate with Alice KB-KDC(A,R1) Alice and Bob communicate: using R1 as session key for shared symmetric encryption

  6. Deffie-Hellman v.s. KDC • Deffie-Hellman • +: no infrastructure support • -: computation load on users • KDC • -: need infrastructure support • -: single bottleneck, single point of failure • +: computation load centered at KDC

  7. + + digital signature (encrypt) K K B B K CA Certification Authorities • Certification authority (CA): binds public key to particular entity, E. • E registers its public key with CA. • E provides “proof of identity” to CA. • CA creates certificate binding E to its public key. • certificate containing E’s public key digitally signed by CA – CA says “this is E’s public key” Bob’s public key certificate for Bob’s public key, signed by CA - Bob’s identifying information CA private key

  8. + + digital signature (decrypt) K K B B K CA Certification Authorities • When Alice wants Bob’s public key: • gets Bob’s certificate (Bob or elsewhere). • apply CA’s public key to Bob’s certificate, get Bob’s public key Bob’s public key CA public key +

  9. A Certificate Contains: • Serial number (unique to issuer) • info about certificate owner, including algorithm and key value itself (not shown) • info about certificate issuer • valid dates • digital signature by issuer

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